This invention relates generally to calcined clay products, and more specifically relates to a calcined kaolin clay pigment and method of manufacture of same. The pigment produced by the method of the invention has a low abrasion and very high opacifying properties when incorporated as a filler in paper products.
In the course of manufacturing paper and similar products, it is well known to incorporate quantizes of inorganic materials into the fibrous web in order to improve the quality of the resulting product. The use of appropriate such fillers vastly improves the opacity and printability of certain types of light weight papers such as newsprint. This aspect of use of calcined kaolin clay pigments is discussed in some detail, for example, Fanselow and Jacobs, U.S. Pat. No. 3,586,523. Other aspects of the presently preferred commercial methods manufacturing calcined kaolin pigments for use particularly as fillers in paper manufacture, are also set forth in the said Fanselow et al patent, as well as in additional United States patents such as McConnell et al, U.S. Pat. No. 4,381,948.
The Fanselow et al and McConnell et al patents are indeed representative of the methodology widely employed in the kaolin industry in order to produce calcined kaolin clay pigments for use in paper manufacturing. Study of these patents will show that the objective of same, as is customary in the art, is to produce a fine particle size calcined kaolin clay pigment of relatively very high brightness, beginning with a crude kaolin which has a relatively very low brightness. A preferred crude feed material for use in processes such as are disclosed in the Fanselow and McConnell patents, is a highly discolored, so-called "gray" kaolin, which is referred to in the Fanselow patent as a "hard sedimentary kaolin clay." Thus, the gray crude which is used in the example of Fanselow has an initial brightness of 78, where the figure cited refers to the so-called G.E. scale. Procedures for measuring brightness as set forth in this application, and as is generally recognized in the industry, are in accord with TAPPI procedure T646os75. As a result of the beneficiation treatment set forth in the Fanselow et al patent, these brightnesses are considerably increased indeed to a very high whiteness. Claim 2 of the Fanselow et al patent thus recites a G.E. brightness within the range of 92% to 95%. Similarly, the McConnell et al patent describes a resultant pigment having a brightness of at least 93 as being the final output product from practice of the beneficiation methods set forth therein. A calcined kaolin pigment substantially produced in accordance with the McConnell et al patent is available commercially from ECC International Inc. of Atlanta, Ga., under the trademark ALPHATEX.RTM..
It may be noted that both the McConnell et al. and the Fanselow et al. patents are concerned with fully calcined kaolins as opposed to metakaolins. When an uncalcined kaolin is heated (i.e. calcined) to about 1098.degree. F. an endothermic reaction occurs. Essentially all of the water of hydration associated with the uncalcined kaolin crystals is eliminated and an essentially amorphous (as measured by X-ray diffraction) material referred to as "metakaolin" results. If the kaolin is heated to higher temperatures, further significant changes occur. The metakaolin undergoes an exothermic reaction (which typically occurs at about 1700.degree. to 1800.degree. F.). Such material is then referred to as a "fully calcined kaolin".
In the McConnell patent, it is emphasized that the crude used to produce the high brightness pigments preferably includes not more than 2% by weight of titanium expressed as TiO.sub.2. A principal reason for this is that clay minerals occurring in nature, including kaolin clays, frequently contain their discoloring contaminants in the form of iron and/or titanium-based impurities. The quantities of the titaniferous impurities in sedimentary kaolins of Georgia are significant and are commonly present as iron oxide-stained titanium oxides. Irrespective of whether calcining is used, it has commonly been considered in the kaolin industry that it is paramount to refine the crude kaolins to bring the brightness characteristics of the resultant product to a level acceptable for various applications such as paper coating, or as mentioned, even for filling. Among the techniques which have been used in the past to remove the discoloring impurities, are the use of hydrosulfites for converting at least part of the iron-based impurities to soluble form, which may then be extracted from the clay. A further method which has come into increasing use in the kaolin industry involves the use of high intensity magnetic separation as described, for example, in such patents as Marston, U.S. Pat. No. 3,627,678. This method is also useful in removing titaniferous impurities in that although titania when pure has little magnetic attractability, the iron-stained titania which forms the basis (as mentioned) for the bulk of discolorants in many kaolins, may often be quite effectively removed by imposition of such a high intensity magnetic field.
One of the further, very effective methods for removing titaniferous impurities including iron oxide-stained titanium oxides, is the froth flotation technique. Generally according to this method, an aqueous suspension or slurry of the clay is formed, the pH of the slurry is raised to an alkaline value and a collector agent is added. The slurry is then conditioned by agitating for a short period. A frothing agent if necessary is added to the conditioned slurry, after which air is passed through the slurry in a froth flotation cell to effect separation of the impurities from the mineral.
Further details regarding the use of froth flotation techniques for removing titanium-based impurities from kaolins may be found at numerous places in the prior art, including for example U.S. Pat. No. 3,450,257 to E. K. Cundy, U.S. Pat. No. 4,518,491 to B. M. Bilimoria, and U.S. Pat. No. 4,090,688 to Alan Nott. In the procedures set forth in these patents, the iron-stained titania "contaminants" are separated with the froth. These separated materials, because of their very high titania content and high degree of discoloration, have in the past simply been termed "rejects," and as such discarded or used for some purpose unrelated to being pigments for paper manufacture. What therefore is to be especially appreciated, is that the discoloring iron-stained titania-based impurities removed from the crude kaolin by froth flotation (or even where such materials have been removed by magnetic separation), have heretofore been regarded as essentially useless material having little or no economic value for proper manufacture. The view in short, has in the past been that the object of beneficiation of kaolins was simply stated to remove these "contaminants" to thereby brighten the output product from which these contaminants had been removed.
Both the brightness characteristics of the given kaolin and the opacifying properties of same when incorporated as a filler in paper, may be quantitatively related to a property of the filler identified as the "scattering coefficient S". The said parameter, i.e., the scattering coefficient S of a given filler pigment, is a property well-known and extensively utilized in the paper technology art, and has been the subject of numerous technical papers and the like. The early exposition of such measurements was made by Kubelka and Munk, and is reported in Z. Tech Physik 12:539 (1931). Further citations to the applicable measurement techniques and detailed definitions of the said scattering coefficient are set forth at numerous places in the patent and technical literature. Reference may usefully be had in this connection, e.g. to U.S. Pat. No. 4,026,726 and U.S. Pat. No. 4,028,173. In addition to the citations set forth in these patents, reference may further be had to Pulp and Paper Science Technology, Vol. 2 "Paper", Chapter 3, by H. C. Schwalbe (McGraw-Hill Book Company, New York).
One of the long-recognized concerns that arises where a kaolin clay is subjected to calcination is the increase in abrasiveness, which can result from the formation of various abrasive phases during the calcination process. The presence of iron in the form e.g. of iron-stained titania, can promote the formation of these undesired phases, and thereby contribute to the unwanted abrasiveness in the calcined product. It is therefore a concern that any process which promotes the presence of high titania content in the calciner feed, could lead to a calcined product having undesirably high abrasion characteristics. Such abrasiveness is detrimental to the principal use of the pigments, since among other things, it effects rapid wear at portions of the paper making apparatus. The generation of abrasive phases is a particularly acute problem where the higher temperatures incident to full calcination are employed.
In accordance with the teachings of U.S. Pat. No. 5,047,375, and contrary to prior experience and practice in the kaolin industry, it was found that a pigment possessing highly useful properties, especially with respect to opacifying light weight paper such as newsprint, may be produced by calcining the very high titania content "rejects" yielded by practice of conventional froth flotation beneficiation. Other "rejects" having similar high content of iron-stained titania, as for example those yielded from high intensity magnetic separation (H.I.M.S.), may also be used in the invention disclosed in the U.S. Pat. No. 5,047,375.
According to such patent, an iron-stained titania-containing kaolin is subjected to froth flotation, after which froth "rejects" comprising kaolin enriched in said iron-stained titania are recovered, then subjected to dewatering and drying, to thereby produce a kaolin enriched in titania. This intermediate is calcined at a temperature in the range of about 1500.degree. to 2200.degree. F. to yield the finished product. The froth "rejects" are not otherwise beneficiated prior to calcining, but are used in their as-is form as recovered from the flotation process. The starting material subjected to the froth flotation is typically a naturally occurring kaolin crude having a titania content of from about 1 to 2 weight %, and the enriched kaolin yielded from the froth flotation process has a titania content of from about 2 to about 15 weight % and preferably has a titania content of at least 5 weight %. Similar considerations apply where H.I.M.S. is the source of the rejects. When the pigment produced in accordance with the U.S. Pat. No. 5,047,375 is utilized in the filling of paper, it imparts a brightness in the range of about 60 to 90, but yields substantially higher opacifying properties than a prior art calcined kaolin not enriched in titania. Yet more preferably, the brightness of the pigment resulting is in the range of about 60 to 80. Pigments having these brightness values had previously been considered undesirable for use as fillers; but in accordance with the U.S. Pat. No. 5,047,375 invention, these fillers were found to produce very high opacifying properties, thereby vastly improving the use and printability of such thin, "low grade" papers such as newsprint.
A related teaching is found in U.S. Pat. No. 5,137,574, where the high opacifying pigment is produced by calcining one or more high titania content kaolin fractions which are separated from a whole crude by size classification, froth flotation, magnetic separation or the like.
In one method of practicing the U.S. Pat. No. 5,137,574 invention, there is separated from one or more iron-stained titania-containing crude kaolins having a titania content of from about 1 to 2 weight percent, one or more titania-enriched kaolin fractions, containing titania in the amount of above 2 weight percent. The one or more enriched kaolin fractions are subjected to wet particulate media grinding. The ground fraction or fractions are dewatered and dried, and then calcined at a temperature in the range of about 1500.degree. to about 2200.degree. F. The products yielded from calcination can then be pulverized to a desired screen size to yield the finished product.
The one or more separated kaolin fractions may include the reject portion from a froth flotation treatment of the iron-stained crude kaolin. Each of the fraction or fractions subjected to media grinding contains over 2% titania. If only a single fraction is used, its titania content will be up to about 3%. If multiple fractions are used, some may have as much as 8% titania; but the relative proportions of the several fractions is such that the total material subjected to the media grinding has a titania content of from 2 to 3%. Preferably the one or more fractions are thus sandground to provide a P.S.D. of sufficient fineness so that at least 90%&lt;2 .mu.m (by weight).
In a preferred procedure, the one or more media ground fractions are blended with a fine particle size unground kaolin prior to calcining. The unground kaolin can e.g. be the recovered product from a centrifuge separation incident to beneficiation of a kaolin crude, and has a titania content of less than 2%. The unground kaolin is already of reduced particle size (e.g. being the fine cut from centrifuging, such as 92%&lt;2 .mu.m), and as is known, will therefore be relatively free of elements which generate abrasive phases upon calcining. The proportion of the media ground fraction or fractions to the unground kaolin is such as to provide a kaolin calciner feed with above 2% to about 3% titania. The one or more fractions may comprise 30 to 50% of the blend, and the unground kaolin comprises the balance.
The use of the aforementioned media grinding step is deemed necessary in the U.S. Pat. No. 5,137,574 in order to reduce the particle size of the separated fraction or fractions, in that by virtue of the use of a titania-enriched fraction, the abrasion of the final calcined product can become unacceptably high. By reducing the particle size of the ground product to a range such that at least 85% (and preferably 90%) by weight are beneath 2 micrometers, the high titania content does not unduly affect the abrasion, while at the same time the opacifying properties of the pigment are substantially retained. The grinding step is preferably accomplished by use of a media such as fine sand, silica, quartz or the like.
The brightness of the high opacifying fully calcined pigment resulting from the process of the U.S. Pat. No. 5,137,574 patent is in the range of about 70 to 82. The Bruening abrasion is less than 42, and the titania content is greater than 2% by weight. The Bruening abrasion is preferably below 35, and may be in the range of 20-35.
To be noted is that brightness values as indicated have typically been considered undesirable for use as fillers; but in accordance with the U.S. Pat. No. 5,074,375 and U.S. Pat. No. 5,137,574, these fillers have been found to produce very high opacifying properties, thereby vastly improving the use and printability of such thin, "low grade" papers such as newsprint.
A further pertinent teaching is found in Berube et al., U.S. Pat. No. 5,011,534. In the method disclosed therein a low abrasion kaolin clay pigment suitable for filling paper is produced from mechanical pulp by selecting a kaolin crude composed of kaolin particles having a low crystallinity index, a G.E. brightness below 82%; a particle size distribution such that at least 75% by weight is finer than 2 microns; the kaolin containing at least 1.2% by weight Fe.sub.2 O.sub.3 and at least 2% by weight TiO.sub.2. The crude is formed into a dispersed aqueous pulp containing at least about 55% clay solids, centrifuged to remove grit, and without subjecting the resulting degritted slip to bleaching or further particle size fractionation is spray dried to produce pulverulent microspheres. The microspheres are pulverized and calcined until the kaolin undergoes at least partially the characteristic kaolin exotherm (in the Example the Samples are heated to 1900.degree. F.), and then repulverized. The resulting calcined clay has a G.E. brightness from 70 to 84%.